Since the turn of the century, major advances have been made in understanding the neuronal control of movement. Rhythmic behaviors such as walking, swimming, flying, and breathing have been particularly amenable to analysis. A common conceptual organization has emerged for the neuronal control of rhythmic movements. There is now nearly universal agreement that the basic motor pattern underlying rhythmic movements is produced by a network of neurons comprising a central pattern generator (CPG). The CPG is activated by a command system and in turn activates the motor neurons. The proposed project is a cellular and mechanistic analysis of how these conceptual functions are implemented by a nervous system. For this purpose we will concentrate on the organization of the central nervous system mediating swimming in the mollusc, Tritonia diomedea. Some questions to be addressed are: What constitutes a command system and how does it activate the motor apparatus? What are the neuronal correlates of a central pattern generator network and how are complex motor patterns produced, maintained, and coordinated? A combined electrophysiological, histological, and pharmacological approach will be taken. The experiments are divided into four sections. First is an identification of the neuronal elements which perform the sensory, command, CPG, and motor functions. Second is a specification of the monosynaptic connectivity within the neuronal network. Third is a biophysical analysis of membrane properties contributing to integration. Fourth is a computer reconstruction and evaluation of the role of network, synaptic, and membrane properties in motor pattern generation. The results should be applicable to other rhythmic motor systems in which this type of analysis is not currently feasible.